The present application relates to a fine particle analyzing apparatus and a fine particle analyzing method by which a sample such as a fine particle is optically detected. In particular, the present application relates to a fine particle analyzing apparatus and a fine particle analyzing method in which semiconductor laser is used as a light source.
Commonly, when physiologically-related fine particles such as cells, microorganisms, and liposome are identified, an optical measuring method using flow cytometry (flow cytometer) is employed (For example, Supervised by H. Nakauchi, “Cell Engineering Additional Volume, Experimental Protocol Series, Freely Flow Cytometry”, Second Edition, Shujunsha Co., Ltd., published on Aug. 31, 2006). Flow cytometry is a method in which fine particles flowing in a flow path in a line are irradiated with laser beams having a specific wavelength and thus fluorescence or scattering light emitted from each of the fine particles is detected so as to singly identify the plurality of fine particles.
Specifically, a laminar flow is formed by sample liquid containing fine particles which are measurement objects and sheath liquid which flows around the sample liquid, so as to line up the plurality of fine particles contained in the sample liquid, in the flow path. When laser beams are radiated to the flow path in such state, the fine particles pass transversely across the laser beams one by one. At this time, fluorescence and/or scattering light which are/is excited by the laser beams and emitted from each of the fine particles are/is detected with an optical detector such as a charge coupled device (CCD) and a photo-multiplier tube (PMT). Then, the light detected by the optical detector is converted into an electric signal to be digitized and statistical analysis is performed so as to determine a type, a size, a structure, and the like of each of the fine particles.
Meanwhile, in order to quantitatively and stably analyze a sample in the flow cytometry described above, it is preferable to constantly maintain the light amount of excitation light (laser beam) radiated to the sample steady. However, a beam spot of excitation light (laser beam) is commonly small such as about several dozen μm, and variation in power density occurs in a three-dimensional direction (an optical axis depth direction and a direction orthogonal to the optical axis) in a beam spot.
Therefore, in related art, a fine particle analyzing apparatus that controls laser driving so as to reduce noise derived from a light source is proposed (refer to Japanese Unexamined Patent Application Publication No. 5-232012, Japanese Unexamined Patent Application Publication No. 9-178645, Japanese Unexamined Patent Application Publication No. 2009-53020, and Japanese Unexamined Patent Application Publication No. 2005-172465). For example, in an apparatus disclosed in Japanese Unexamined Patent Application Publication No. 5-232012, a single-mode oscillation type semiconductor laser is used as a light source, laser current is controlled so as to stabilize an output of a light amount sensor built in the laser, and a preset temperature in temperature control of the laser is switched when mode hop is detected.
In an apparatus disclosed in Japanese Unexamined Patent Application Publication No. 9-178645, a laser diode which is a light source is driven by a laser driving circuit which outputs driving current obtained by superimposing a high-frequency component on direct current, so as to turn a longitudinal mode of the laser diode to multimode. In an apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2009-53020, amplitude of a high-frequency wave outputted from a high-frequency wave superimposing circuit is controlled depending on intensity of direct current which is outputted from a direct current driving circuit, so as to multimode-oscillate a laser diode. Further, in an apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2005-172465, a high-frequency current component from an oscillator is superimposed on driving current of a semiconductor laser so as to make an oscillation central wavelength of the semiconductor laser follow a resonance wavelength.